The present invention generally concerns a method of measuring a welding voltage at a heating coil of a heating coil fitting and a heating coil welding apparatus which can be used for carrying out the method.
In order to sealingly connect installation components of plastic material, such as for example pipes and tubes, use is made in a practical context of heating coil fittings. Heating coil fittings comprise a fitting tube which is made from plastic material, with at least one heating coil disposed in the wall of the tube. The heating coil can be connected to an electrical welding current supply unit by way of two connecting ends which are taken to the outside of the fitting tube and which terminate in connecting elements. For the purposes of welding the two installation tubes of plastic material, they are introduced into the fitting tube which is of an inside diameter that is slightly larger than the outside diameter of the two tubes which are to be connected together. An electrical current is then supplied to the heating coil. In that case, the energy supplied is converted into heat as a consequence of the electrical resistance inherent in the heating coil. The heat produced in that way, originating from the heating coil, penetrates slowly through the plastic materials of the fitting tube and the installation tubes to be welded together. When the softening point or the melting range of the materials involved is exceeded, the materials plasticise or soften and flow together. When the materials then cool down they harden to form a connection between the two tubes and the fitting, which affords gas-tight and liquid-tight sealing integrity. The terms xe2x80x98heating coil weldingxe2x80x99 or just xe2x80x98weldingxe2x80x99 are also frequently employed to denote that procedure.
In one form of a heating coil welding procedure, the welding voltage which is applied to the heating coil is kept constant over the duration of the welding process by way of the welding current supply unit. In practice the welding voltage is generally not measured at the equipment end, that is to say at the ends of the connecting lines at the welding current supply unit, but by way of a separate measurement line, at a position adjacent the heating coil, at the connecting elements which are mounted to the ends of the connecting lines connecting the heating coil to the welding current supply unit. That obviates involvement of the voltage drop at the connecting lines, in order to determine the actual voltage at the heating coil, at the end of the connecting lines connected thereto.
The quality of a welded joint can be seriously reduced by virtue of dirty or worn junction elements or the contacts disposed therein, as then a part of the voltage is dropped off at the resistance to current flow at the contact. A contact with a higher resistance in that way is heated up to a much greater degree than a contact involving a lower level of resistance. At the same time the fitting receives a level of heating output which is reduced by that which is dropped off at the contact, and which reduces the quality of the weld produced.
The cause of this is that the welding current supply unit or a voltage measuring unit disposed therein measures or regulates the welding voltage at the junction elements of the connection lines, with which the welding current supply unit is connected to the actual connecting elements of the heating coil fitting, and thus cannot take appropriate account of the resistance to current flow at the contact. Accordingly, an excessively low voltage occurs at the heating coil of the fitting, depending on the respective degree of wear or contamination of the contacts in the respective junction elements of the connecting lines or connecting elements of the heating coil fitting. Contamination at the contacts can be for example dirt, dust or plastic material which has flowed thereinto, but also an oxidation layer which is formed on the contact of the junction elements and/or the connecting elements of the heating coil fitting.
Other control procedures in this respect make use of the power or the current involved, as a regulating parameter, or follow fixedly predetermined regulating curves. In any case the precise value of the welding voltage applied to the heating coil is always required.
A further method of measuring the welding voltage at the heating coil in this respect provides for direct measurement of the temperature of the heating coil by means of the temperature dependency of the material of the heating coil. It will be apparent that, to achieve that, the resistance of the heating coil must be measured with a high degree of accuracy prior to and during the welding operation. An excessively high level of resistance to current flow, which is moreover temperature-dependent, means however that such a regulating procedure is generally excessively inaccurate.
Finally, in a practical context, it is also possible to fit a measurement tip or probe to a contact of one of the two connecting elements at the heating coil fitting, by which it is connected to the welding current supply unit, in monitoring the heating coil welding procedure. Incorporated in that contact of the heating coil fitting is a reference resistance which can be read off with the measuring tip. The resistance value ascertained in that way is converted by way of a table into a welding time. The measuring tip is therefore in this case a part of an automatic system for reading off the welding parameters.
An object of the present invention is to provide a method of measuring a welding voltage at a heating coil of a heating coil fitting, which permits the welding voltage applied to the welding coil to be precisely detected, independently of influencing parameters such as corrosion and the like.
Another object of the present invention is to provide a method of measuring a welding voltage at a heating coil fitting, which can provide for precise monitoring and control of the welding voltage in such a way as to optimise the welding procedure.
Yet another object of the present invention is to provide a heating coil welding apparatus which is designed to precisely ascertain the welding voltage at the heating coil to afford reliable control of the welding procedure.
In accordance with the principles of the present invention in the method aspect the foregoing and other objects are attained by a method of measuring a welding voltage at a heating coil of a heating coil fitting, in which a welding current supply unit is connected by way of connecting lines and junction elements to the connecting elements of the heating coil fitting. The welding voltage is then detected by means of a measurement unit. The welding voltage is taken off at a respective contact of the two connecting elements of the heating coil fitting electrically insulatedly from the connecting lines and the junction elements.
As will be noted from the description hereinafter of a preferred embodiment of the invention, the fact that the welding voltage is taken off at a respective contact of each of the two connecting elements of the heating coil fitting means that influences which are caused for example by corrosion at the contact surfaces in the junction elements of the welding current supply unit can have no effect. In that way the welding voltage at the heating coil can be reliably and accurately ascertained and it is thus possible to achieve an optimum welding process. In order to exclude influences arising out of the connecting lines and the junction elements when measuring the welding voltage, it is advantageous if the measurement procedure is implemented in electrical isolation from the connecting lines, with the measuring elements being electrically insulated from the connecting lines and the junction elements respectively.
Detecting the welding voltage at the contact of the connecting elements and detecting a further voltage value at the connecting lines affords the possibility of detecting the drop in voltage as a result of the transfer resistance or resistance to current flow at the contacts.
In order to ensure reliable contact in terms of the measurement procedure in a preferred feature of the invention the welding voltage is detected by means of measurement elements which are reversibly acted upon by a force in the direction of the respective contact of the connecting element. For example that force can be a spring force. That affords the possibility of the measurement element being able to penetrate a deposit which may have formed on the respective contact of the connecting elements.
It may further be found to be advantageous if the measurement operation is effected at least approximately in a current-less mode as the transfer resistance is dependent on the contact surface area and the measurement current passing through the arrangement. Thus, even in the case of a small contact surface area, the transfer resistance is vanishingly low with a low current while it assumes a crucial value with the same contact surface area and a high current, for example the welding current, by virtue of the high current density in relation to surface area. Approximately current-less measurement can be effected for example by the input resistance of the measurement unit being high.
Further in accordance with the principles of the invention in the apparatus aspect a heating coil welding apparatus, for example for carrying out the method according to the invention, comprises at least one welding current supply unit and at least first and second connecting lines which are connected to the welding current supply unit and which each have a respective junction element. The welding current supply unit can be releasably connected by way of the connecting lines to connecting elements of at least one heating coil fitting. The apparatus further includes a measurement unit for detecting at least the welding voltage, a respective first measurement line going from the measurement unit to the junction element of each one of the connecting lines. Each junction element has at least one measurement element which is respectively connected to the first measurement line and which can be brought into electrically conductive contact with a contact of a connecting element of the heating coil fitting to detect the welding voltage in electrically insulated relationship from the respective connecting line and junction element.
As a further measure for ensuring that good contact is made between the measurement element and the contact of the respective connecting element, in a preferred feature of the invention the measurement element may have a measurement tip which is adapted to be brought into contact with the contact of the connecting member. That measuring tip assists with penetration of any deposits which may be present at the contacts of the connecting elements.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of a preferred embodiment.